Patent document 1 discloses a conventional roller type speed reducer of this type. FIGS. 21 and 22 show the roller type speed reducer disclosed in Patent document 1. This roller type speed reducer includes an input shaft 60 and an output shaft 61 arranged coaxial with each other with their ends facing each other. Their ends facing each other are covered by a housing 62. The housing 62 supports an internal gear 63 having internal teeth 63a with a curved profile on its radially inner surface. Axially spaced apart eccentric disks 64 are mounted on the end portion of the input shaft 60 so as to be located radially inwardly of, and rotatable relative to, the internal gear 63. The output shaft 61 has a cage 66 at its end disposed between the internal gear 63 and bearings 65 press-fitted on the radially outer surfaces of the respective eccentric disks 64. The cage 66 is formed with two rows of pockets 67, each row of pockets being circumferentially equidistantly spaced apart from each other and radially facing one of the eccentric disks 64. The pockets of each row are fewer in number than the internal teeth 63a. A roller is 68 is received in each pocket 67 so as to be rollable along the radially outer surface of one of the bearings 65.
In operation of this speed reducer, when the input shaft 60 rotates, the eccentric disks 64 rotate together with the input shaft 69. This causes each roller 68 to engage the internal teeth 63a of the internal gear 63 one after another. In particular, when the input shaft 60 rotates once, each roller 68 moves circumferentially by a distance equal to the circumferential dimension of one internal tooth 63a. This results in the rotation of the output shaft 61 at a reduced speed relative to the input shaft 60.
For smooth rotation, the internal teeth 63a have to be shaped such that all of the rollers 68 are always in contact with internal teeth 63a. Specifically, each tooth 63a has a profile equal to the curve at the outside of one roller 68 parallel to the locus of the roller 68 when the output shaft 61 is rotated by an angle corresponding to one pitch of the internal gear 63 by the eccentric disks 64.